The present invention relates to an image intermediate-frequency processing apparatus which makes it possible to receive an FM broadcast in a television or a VTR.
In recent years, portable televisions such as liquid crystal televisions, color televisions attached with VTRs or the like have become very popular. There have also come in the market television receivers capable of receiving ordinary FM broadcast as well as television broadcast. FIG. 3 is a block diagram that shows a schematic configuration of a prior-art image intermediate-frequency processing apparatus that is built into such a television receiver.
The image intermediate-frequency processing apparatus shown in FIG. 3 can broadly be divided into three sections, i.e. a tuner section, an intermediate-frequency filtering section and an image intermediate-frequency processing section. As shown in FIG. 3, the tuner section is structured by an antenna 1 that receives broadcasting waves, and a tuner 2 that selects a desired broadcasting wave from the waves received by the antenna 1. The tuner 2 also converts the selected television broadcasting signal into an intermediate frequency signal including an image i.e., video modulation component and a voice i.e., audio modulation component when receiving a television broadcast. Further, in order to make it possible to receive FM broadcast as well a television broadcast, during reception of FM broadcast, the tuner 2 converts the selected FM broadcasting signal into intermediate-frequency signal of the same frequency as that of a voice intermediate-frequency component received during the reception of a television broadcast. The tuner 2 then outputs this converted intermediate-frequency signal.
The intermediate-frequency filtering section includes an image intermediate-frequency SAW filter 3 (hereinafter to be referred to as VIF-SAW filter) which extracts an image intermediate-frequency signal of 58.75 MHz (in the case of Japan) from the output of the tuner 2, and a voice intermediate-frequency SAW filter 4 (hereinafter to be referred to as SIF-SAW filter) which extracts a voice intermediate-frequency signal of 54.25 MHz (in the case of Japan) from the output of the tuner 2.
The image intermediate-frequency processing section can be further divided into an image signal processing section and a voice signal processing section. The image signal processing section includes a VIF amplifier 5 (hereinafter to be referred to as VIF-AMP) which amplifies the output of the VIF-SAW filter 3, an image detector 6 which detects the output of the VIF-AMP 5, an IF automatic gain control circuit 7 (hereinafter to be referred to as IF-AGC circuit) which controls the gain in the VIF-AMP 5 according to the output of the image detector 6, an IF-AGC filter 8 made up of a capacitor, an automatic phase detector 11 (hereinafter to be referred to as APC detector) which compares phase of the output signal of the VIF-AMP 5 with phase of an output signal of a voltage control oscillator (hereinafter to be referred to as VCO) 12, outputs a signal that represents a phase difference between these two phases to the VCO 12, and carries out automatic phase control to the VCO 12, and an APC filter 14.
The APC filter 14 is generally structured by a capacitor and a resistor. An IF-AGC filter terminal 9 is a terminal that connects the IF-AGC filter 8 to an output of the IF-AGC circuit 7. An APC filter terminal 13 is a terminal that connects the APC filter 14 to an output of the APC detector 11.
The voice signal processing section is structured by an SIF detector 16 that receives the outputs of the SIF-SAW filter 4 and VCO 12, carries out SIF detection and obtains a signal of 4.5 MHz that is an inter-carrier, and an FM detector 17 (hereafter to be refereed to as FM-DET) which FM-detects the output of the SIF detector 16 and converts the detected wave into a voice signal. The voice signal is output from a voice signal output terminal 18.
Further, there are provided a switch 10 that is changed over to a TV terminal side so as to open both sides of the IF-AGC filter 8 at the time of receiving a television broadcast and that is changed over to an FM terminal side so as to short-circuit both sides of the IF-AGC filter 8 at the time of receiving an FM broadcast, and a switch 15 that is changed over to the TV terminal side so as to supply the output of the APC detector 11 to the VCO 12 at the time of receiving a television broadcast and that is changed over to the FM terminal side so as to oscillate the VCO 12 in the free running oscillation frequency at the time of receiving an FM broadcast. Further, for the image intermediate-frequency processing apparatus to function as a color television receiver, although not shown here, there are provided a color signal processing section and a luminance signal processing section, etc., in addition to the above-described structure, in the image intermediate-frequency processing section.
The operation of the prior-art image intermediate-frequency processing apparatus will be explained next. The case of receiving a television broadcast is explained first. In this case the switch 10 and the switch 15 are changed over to the TV terminal side. When the antenna 1 receives a signal sent from a broadcasting station, the received broadcasting signal is mixed with a local oscillation output corresponding to a desired channel selected by the tuner 2, and is input into the filtering section (VIF-SAW 3 and IF-SAW 4).
The VIF-SAW 3 takes out only the image intermediate-frequency signal from the signal obtained from the tuner 2, and inputs this image intermediate-frequency signal into the VIF-AMP 5 at the next stage. The VIF-AMP 5 amplifies the image intermediate-frequency signal obtained from the VIF-SAW 3 to a constant level. The image intermediate-frequency signal output from the VIF-SAW 3 is detected and demodulated by the image detector 6. The demodulated image signal is output from an image signal output terminal 19. In this case, as the switch 10 is in the status of not short-circuiting either end of the IF-AGC filter 8, the IF-AGC circuit 7 can input into the VIF-AMP 5 an AGC voltage generated based on the image-detected output of the image detector 6.
The AGC voltage obtained from the IF-AGC circuit 7 is smoothed by the IF-AGC filter 8. The smoothed AGC voltage is input into the VIF-AMP 5. As explained above, when receiving a television broadcast, a negative feedback loop is formed by the VIF-AMP 5, the image detector 6, the IF-AGC circuit 7, and the IF-AGC filter 8.
A reference carrier to be input into the image detector 6 for detecting an image is generated by a phase-locked loop (hereinafter to be referred to as a PLL) formed by the APC detector 11, the APC filter 14 and the VCO 12 that are closed by the switch 15. In other words, a reference carrier of which phase is aligned with the phase of the carrier of the VIF signal by the VCO 12, is input into the image detector 6.
Thus, when receiving a television broadcast, the VIF-AMP 5 can maintain the amplification operation at a constant level, by changing over the switch 10 and the switch 15 to the TV terminal side respectively. Resultantly, it is possible to obtain a desired image signal corresponding to a selected channel, from the image signal output terminal 19.
Regarding a voice signal of the television broadcast, at first, the SIF-SAW 4 takes out only the voice intermediate-frequency signal (54.25 MHz in the case of Japan) from the signal obtained by the tuner 2. This voice intermediate-frequency signal is then input into the SIF detector 16. The SIF detector 16 multiplies the voice intermediate-frequency signal obtained from the SIF-SAW filter 4 by a reference carrier obtained from the VCO 12, thereby to carry out the wave detection. Further, this voice intermediate-frequency signal is converted into the FM signal of which carrier frequency is 4.5 MHz. The FM signal obtained from the SIF detector 16 is input to the FM detector 17. The FM detector 17 demodulates a voice signal from this FM signal, and outputs the voice signal to the signal output terminal 18. Thus, it is possible to obtain a desired voice signal corresponding to a selected channel.
On the other hand, when receiving FM broadcast, switches 10 and 15 are changed over to the FM terminal side. Thus, both ends of the IF-AGC filter 8 are short-circuited, and the gain in the VIF-AMP 5 is set to a minimum. When the gain of the VIF-AMP 5 is minimum, no signal is output to the APC detector 11. Further, as the switch 15 has been changed over to the FM terminal side, the VCO 12 oscillates in the free running oscillation frequency without receiving an influence of the APC detector 11. The FM broadcasting signal obtained from the tuner 2 is input into the SIF-SAW 4. The SIF-SAW 4 takes out only the voice intermediate-frequency signal of 54.25 MHz from this FM broadcasting signal.
The voice intermediate-frequency signal output from the SIF-SAW filter 4 is input into the SIF detector 16 for detecting the signal. In this case, the SIF detector 16 multiplies the oscillation output that is in free running oscillation in the VCO 12 by the input voice intermediate-frequency, so that an FM signal of which carrier frequency is 4.5 MHz is obtained. The FM signal obtained from the SIF detector 16 is input into the FM detector 17 in a similar manner to that of the above-described case of receiving a television broadcast. The FM detector 17 demodulates a voice signal from the FM signal and outputs the voice signal to the voice signal output terminal 18. Thus, it is possible to obtain a desired voice signal corresponding to the selected channel.
However, the prior-art image intermediate-frequency processing apparatus capable of receiving an FM signal has the following problem. As the apparatus uses a free running oscillation frequency of the VCO 12 in the FM wave detection when receiving FM broadcast, the FM wave detection is directly affected by a variance in the manufacturing of the VCO 12 or a variation in the temperature-dependent characteristics of the VCO 12.
The SIF detector 16 multiplies the output of the free running oscillation VCO 12 by the voice intermediate-frequency signal for obtaining a carrier frequency. Therefore, the carrier frequency shall deviate from the desired frequency (4.5 MHz) if there is a variance in the free running oscillation frequency of the VCO 12. As a result, there is a problem that the quality of the voice signal is deteriorated when receiving FM broadcast.
It is an object of the present invention to provide an image intermediate-frequency processing apparatus capable of receiving FM broadcast that can stabilize the free running oscillation frequency of a VCO with non-adjustment during the reception of FM broadcast and that can make the quality of the sound signal during the reception of FM broadcast equal to the sound quality during the reception of television broadcast.
In order to achieve the above object, according to one aspect of the present invention, in the image intermediate-frequency processing apparatus, a phase-locked loop, i.e. a second phase-locked loop is exclusively provided which performs intermediate-frequency processing of FM broadcasting signal.
Further, the second phase-locked loop has a second voltage control oscillating unit which oscillates according to an input of a second control voltage, a first frequency-dividing unit which divides the frequency of the output of the second voltage control oscillating unit, a reference signal generating unit which generates a reference signal, a second frequency-dividing unit which divides the frequency of the reference signal, a second phase-comparing unit which compares a phase of the output of the first frequency-diving unit with a phase of the output of the second frequency-dividing unit, and inputs a result of the comparison to the second voltage control oscillating unit as the second control voltage, and a second filter which smoothes the second control voltage. Thus, this phase-locked loop can perform FM wave detection.
Further, the switching unit inputs an oscillation output of the first voltage control oscillating unit into the voice intermediate-frequency detecting unit at the time of receiving a television broadcast, and inputs an oscillation output of the second voltage control oscillating unit into the voice intermediate-frequency detecting unit at the time of receiving an FM broadcast. Therefore, it is possible to discriminate the use of oscillation signals to be input into the voice intermediate-frequency detecting unit between the time of receiving a television broadcast and the time of receiving an FM broadcast.
Further, a result of the comparison by the second phase-comparing unit is also input into the first voltage control oscillating unit. Therefore, it is possible to carry out a stable oscillation by absorbing a variance in the free running oscillation frequency of the first voltage control oscillating unit due to the manufacturing or the temperature-dependent characteristics of the first voltage-control oscillating unit, regardless of the time of receiving a television broadcast or the time of receiving an FM broadcast.
Further, the switching unit inputs an oscillation output of the first voltage control oscillating unit into the first frequency-dividing unit without inputting the oscillation output of the second voltage control oscillating unit into the first frequency-dividing unit when receiving FM broadcast. Further, a result of the comparison by the second phase-comparing unit is input into the first voltage control oscillating unit. Therefore, it is possible to discriminate the use of oscillation signals to be input into the voice intermediate-frequency detecting unit between the time of receiving a television broadcast and the time of receiving an FM broadcast.
Further, the frequency dividing ratios in the first frequency-dividing unit and the second frequency-dividing unit can changed based on a program. Therefore, it is also possible to correspond to a plurality of different image intermediate-frequency signals.